In the installation of a blind rivet, the combination of a tubular fastener body or sleeve with a cylindrical stem therein is inserted through a hole in two or more stacked workpieces. The sleeve typically has a preformed head on one end which engages one side of the combined workpiece and has a plain tubular configuration on its tail end protruding from the opposite side of the workpiece. The stem usually has an enlarged head which engages the end of the fastener sleeve and has an elongated portion protruding beyond the head end of the sleeve to be gripped by an installation tool. The stem is pulled by the tool so that its enlarged head deforms the tail of the sleeve, clamping the workpieces together. When the sleeve is thus set, further pulling on the stem causes it to break flush with the exterior of the sleeve head. Various fasteners perform the workpiece clamp-up aspect with varying degrees of effectiveness.
It is desirable that the portion of the stem extending through the sleeve remain captured in the sleeve in that the stem provides additional shear strength to the fastener, and in many installations it is not acceptable to have loose rivet stems confined within an inaccessible portion of the structure. In a three-piece blind fastener, a locking ring fits into a mating annular groove between the stem and the sleeve to lock the stem in the sleeve. Such a fastener is widely used and has many advantages, one advantage being that a given fastener can accommodate workpieces within a range of workpiece thicknesses in that the stem is wire drawn until a positive stop is reached. These fasteners have also been effective for tightly clamping the workpieces together.
It is desirable that a two-piece fastener, which is naturally less expensive, also provide good clamp-up capabilities, as well as good stem retention. Various attempts have been made at attaining these goals, but only partial success has been obtained.
U.S. Pat. No. 3,047,181--Heidenwolf discloses a two-piece blind fastener having an enlarged tail on its stem which appears to have a frusto-conical forward surface and a spherical rear surface. Such fastener appears to draw the workpieces tightly together, and the patent indicates that ... "the sleeve tail flows over the smooth surface of the terminal, spherical portion of the mandrel or stem and encloses the enlarged tail." However, there is further provided teeth at the extreme outer end of the sleeve to facilitate enclosing of the stem tail.
U.S. Pat. No. 3,073,205--discloses a fastener that appears to provide good workpiece clamp up utilizing an enlarged stem head having a tapered leading surface and the enlarged stem head is captured in the tail of the sleeve. However, the sleeve has an annular groove formed on its inner surface to provide the retention capability. Such a groove, of course, adds to the cost of the structure.
U.S. Pat. No. 4,211,145--Dolch discloses a blind rivet having a stem with an enlarged cone-shaped head, and U.S. Pat. No. 4,236,429--Dolch discloses a spherical tail. In each case, the heads are said to be captured within the end of the sleeve. However, these fasteners are made of thermoplastic material and an inwardly extending annular bead is formed on the extreme end of the sleeve to capture the stem head. It is said that this is accomplished because the thermoplastic material tends to return to its original shape after deforming pressure has been relaxed. For many applications, thermoplastic fasteners are, of course, not acceptable.
A two-piece blind fastener made by the Cherry Division of Textron Inc., and referred to as an "N-rivet," employs a stem having an enlarged head with a generally flat forward surface, a curved convex rear surface and a pair of opposing, generally flattened sides giving the enlarged head a somewhat elliptical cross-sectional shape. When a head with such shape enters the end of the sleeve, the sleeve tail cross section is deformed by being enlarged in the longer dimension of the stem head with the result that the adjacent sides of the sleeve tail are drawn inwardly and tend to curl over the end of the stem head, capturing it within the end of the sleeve. While such fastener is relatively inexpensive and is widely used for many applications, a better, more consistent clamp up of the workpieces is desirable. This is particularly so if such result can be obtained without significant increased cost.
U.S. Pat. Nos. 3,850,021 and 3,880,042--Binns disclose the use of axially spaced, outwardly extending, diametrically and axially spaced flanges on a fastener stem, and such flanges deform the end of a sleeve into non circular shapes. However, it appears difficult to manufacture.
U.S. Pat. No. 3,136,204--Reynolds discloses a two-piece blind fastener wherein a sleeve tail is deformed into a so-called tulip shape by a pin head having a conical surface and a crowned rear surface. The sleeve tail provides some limited stem retention capability, but apparently not enough because the sleeve head is also swaged into grooves on the stem.
U.S. Pat. No. 2,558,623--Keating discloses a two-piece rivet assembly wherein a sleeve has a head section of reduced diameter and sleeve material is swaged into a groove in the stem for stem retention by a larger diameter pin section.
A need still exists for an improved, low cost, two-piece blind fastener having the capability to provide excellent workpiece clamp up, plus good stem retention capability.